This invention relates to water propulsion devices and more particularly to enclosed shield marine propellers of one unit construction for augmenting propeller thrust and/or guarding the propeller against underwater obstruction.
It is the common practice to propel watercraft by means of the reaction obtained by the engagement of propeller blades with water. Conventional propeller blades depend on the angular blade action on the water to propel the watercraft. While forward movement of the watercraft is obtained in this manner, forward force efficiency is low. The radial movement of the blades about the longitudinal shaft causes water to be thrown off centrifugally. The centrifugal effect means that force is being exerted radially, thereby reducing forward, longitudinal force. The force intended for forward propulsion is wasted by the blades radially and tangentially agitating the surrounding water which in turn causes cavitation.
Cavitation is the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure caused by the axial movement of the blades. Specifically, it is an aeration phenomena which causes the blades to lose their "grip" on the water. Cavitation is a result of the propeller blades spinning so fast that the water pressure against the blades decreases so as to actually make the water boil. As the bubbles of water collapse on the surface of the blades the metal blades can be eaten away, and this is in fact a frequent cause of structural damage to propellers. Forward propulsion is lost because thrust is directly proportional to the blades "gripping" efficiency.
Many serious accidents have occurred to persons water skiing or swimming in the vicinity of the exposed propellers of water craft. Furthermore, the waves created by the exposed propeller's radial water churning, are hazardous to nearby light craft, such as sail boats, canoes, and the like. Exposed propellers are also at risk from rocks and such lying in shallow water, which cause breakage of shear pins or damage to the propeller itself.
Although shrouded propellers have commonly been used for vessels in order to improve thrust, most involve fitting a ring nozzle around a propeller and use bulky bolt-on devices which usually only provide marginal improvements in thrust. The propeller blades of these type devices are not usually connected directly to the shroud. The shroud is commonly bolted to the out drive and is static with respect to propeller movement. The space between the rotating propeller blades outer radial edges and interior of the static shroud prevents good water flow and creates water friction. A common result of this arrangement is "burnt" ends of the propeller blades.
Although basket-like guards have been used for years by boaters in bayous and swamps to keep weeds and grass from fouling props, few is any safety propeller guards are currently being marketed. Those guards which are available, typically plastic cylinders with metal mountings, could reduce boat speeds by as much as twenty-five to thirty percent.